1. Field of the Invention
The invention relates to an armature used in an electromagnetic valve or the like, and an armature driving device.
2. Description of the Related Art
In the past, a metering valve has been provided in a fuel supply systems for an internal combustion engine, for example, to regulate the fuel quantity sent to the internal combustion engine. This metering valve includes a valve body provided in a fuel passage through which fuel flows, and an actuator that changes the fuel flow area of the passage by displacing the valve body.
For this type of actuator, it is conceivable to use an actuator such as that proposed in Japanese Patent Laid-Open Publication No. 10-246169, for example, i.e., an actuator that moves an armature, which is of magnetic material and which is coupled to a valve body so that it can move together with the valve body, in a reciprocating manner by electromagnetic force. A metering valve to which such an actuator has been applied will be described with reference to FIGS. 12 to 14.
FIG. 12 is an expanded cross-sectional view showing a part of the actuator in the metering valve. As shown in the figure, an actuator 91 is provided with a cylindrical armature 93 in a housing 92 of the actuator 91, and an electromagnetic solenoid 94 for imparting an electromagnetic force on the armature 93. The armature 93 is coupled to a shaft 95 that passes through the housing 92 and into a fuel passage 96. The armature 93 is also coupled to a valve body, not shown, in the fuel passage 96 via this shaft 95.
The armature 93, the shaft 95, and the valve body are all positioned on the same axis, and are energized in the direction in which the shaft 95 enters the housing 92 by a spring, not shown, provided on the fuel passage 96 side. Further, the armature 93 is slid in the direction that the shaft 95 protrudes from the housing 92 against the spring force of the spring by electromagnetic force generated when the electromagnetic solenoid 94 is energized. As the armature 93 slides, it displaces the valve body in the fuel passage 96 such that the fuel flow area in the fuel passage 96 changes. When the electromagnetic solenoid 94 is de-energized, the spring force of the spring keeps the valve body in a position in which the fuel flow area in the fuel passage 96 is greatest (i.e., in a fully open position).
Also, the fuel in the fuel passage 96 is sent, through a portion through which the shaft 95 passes, into the housing 92 to provide lubrication. Therefore, the housing 92 fills up with fuel which lubricates a small gap δ between the armature 93 and the housing 92 when the armature 93 slides. When the spaces on both sides in the axial direction of the armature 93 inside the housing 92 are filled with fluid, however, that fluid impedes the sliding of the armature 93 in the axial direction.
To prevent this, a communicating hole 99 extending parallel with the shaft 95 is formed in the armature 93 to provide communication between the two fluid chambers 97 and 98. This communicating hole 99 enables fuel to pass between the two fluid chambers 97 and 98 as the armature 93 slides, thereby minimizing the chance of the fuel in the fluid chambers 97 and 98 impeding the movement of the armature 93 in the axial direction. FIG. 13 is a cross-sectional view of the armature 93 in the radial direction, and FIG. 14 is a cross-sectional view of the armature 93 shown in FIG. 14 as viewed from the direction of arrow A—A.
Forming the communicating hole 99 in the armature 93 as described above makes it possible to minimize the resistance from the fluid inside the fluid chambers 97 and 98 that is generated when the armature 93 slides in the axial direction.
However, in the aforementioned publication, no consideration is given to rotating the armature 93 around its axis. Therefore, it is implausible that the armature 93 would rotate around its axis on its own. Accordingly, the same portions of the armature 93 and the housing 92 would always slide against one another when the armature 93 slides in the axial direction.
In many cases, the sliding surface of the armature 93 in the housing 92 is coated with a coating to reduce friction during sliding. However, when the same portions of the armature 93 and the housing 92 always slide against one another, the coating on those portions may peel off.
If this happens, the coating material that has peeled off may wear down the sliding surface of the housing 92 in the armature 93 at, for example, the hatched portion C in FIG. 12 when the armature 93 slides in the axial direction while that peeled-off coating material is between the armature 93 and the housing 92.